The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes drilling equipment situated at surface and a drill string extending from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. In addition to the conventional drilling equipment mentioned, the system also relies on some sort of drilling fluid system, in most cases a drilling “mud” which is pumped through the inside of the pipe, which cools and lubricates the drill bit and then exits out of the drill bit and carries the rock cuttings back to surface. The mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore which can potentially cause a blow out at surface.
Directional drilling is the process of steering a well away from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is the bottom-hole-assembly (or BHA) which comprises of 1) drill bit; 2) steerable downhole mud motor of rotary steerable system; 3) sensors of survey equipment (Logging While Drilling (LWD) and/or Measurement-while-drilling (MWD)) to evaluate downhole conditions as drilling progresses; 4) equipment for telemetry of data to surface; and 5) other control process equipment such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a string of metallic tubulars (drill pipe). MWD equipment is used to provide downhole sensor and status information to surface in a near real-time mode while drilling. This information is used by the rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, hydrocarbon size and location, etc. This can include making intentional deviations from the planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real time data MWD allows for a relatively more economical and more efficient drilling operation.
In MWD, the currently used MWD tools contain essentially the same sensor package to survey the well bore but send the data back to surface by various telemetry methods. Such telemetry methods include but are not limited to the use of hardwired drill pipe, acoustic telemetry, fibre optic cable, Mud Pulse (MP) Telemetry and Electromagnetic (EM) Telemetry.
EM Telemetry involves the generation of electromagnetic waves which travel through the wellbore's surrounding formations, with detection of the waves at surface. The BHA metallic tubular is typically used as the dipole antenna for the EM telemetry tool by dividing the drill string into two conductive sections by an insulating joint or connector (“gap sub”) typically placed within the BHA, with the bottom portion of the BHA and the drill pipe above each forming a conductor for the dipole antenna. In EM telemetry systems, a very low frequency alternating current is driven across the gap sub. The sub is electrically isolated (‘nonconductive”) at its center joint, effectively creating an insulating break (“gap”) between the very bottom of the drill string and the larger top portion that includes all the drill pipe up to the surface. The low frequency AC voltage and magnetic reception is controlled in a timed/coded sequence to energize the earth and create a measurable voltage differential between the surface ground and the top of the drill string. The EM signal which originated across the gap is detected at surface and measured as a difference in the electric potential from the drill rig to various surface grounding rods located about the lease site.
Advantageously, an EM system can transmit data without a continuous fluid column; hence it is useful when there is no mud flowing. This is advantageous because the EM signal can transmit the directional survey data while the drill crew is adding new pipe.
However, EM transmissions can be strongly attenuated over long distances through the earth formations, with higher frequency signals attenuating faster than low frequency signals, and thus EM telemetry tends to require a relatively large amount of power so that the signals can be detected at surface.
MWD telemetry methods rely on modulation of digital signals similar to that developed in the telecommunications industry. Typically, the signal is modulated by a variety of standard modulation techniques. The three key parameters of a periodic waveform are its amplitude (“volume”), its phase (“timing”) and its frequency (“pitch”). Any of these properties can be modified in accordance with a low frequency signal to obtain the modulated signal. Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information. Amplitude shift keying (ASK) conveys data by changing the amplitude of the carrier wave; Phase-shift keying (PSK) conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). It is known to combine different modulation techniques. For example, combining Amplitude and Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, both the amplitude and the phase of a reference signal (or the carrier wave). Asymmetric Phase-shift keying, (APSK), combines both Amplitude-shift keying (ASK) and Phase-shift keying (PSK) to increase the symbol-set.
The choice of modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal.